THE  MICROSTRtCTtRC  CF  \ 
WOOD  PULP  EIBER 

October  1928 


school  of  mm  mm 
mm  §  ww 


No.  R911 


UNITED  STATES  DEPARTMENT  OF  AGRICULTURE 

FOREST  SERVICE 

FOREST  PRODUCTS  LABORATORY 

Madison,  Wisconsin 

In    Cooperation    with    the    University    oi   Wisconsin 


THE  MICROSTRUCTURE  OF  A  WOOD  PULP  FI3SR* 


By 


GEORGE  J.  RITTER,  Chemist  ' 
and 
G.  H.  CHIDESTSR,  Assistant  Engineer 


The  results  presented  in  this  paper  were  obtained  in  fundamental 
study  that  the  Forest  Products  Laboratory  is  making  in  its  investigation  of 
the  behavior  of  wood,  largely  for  the  purpose  of  bettering  the  utilization 
of  wood.  These  results  will  here  be  considered  in  connection  with  the  prob- 
lems that  confront  the  pulp  and  paper  manufacturer. 


I, Constituents  of  Wood 


Constituents  Commonly  Wasted 

The  manufacturer  of  pulp  and  -paper  naturally  is  interested  in  the 
nature  and  the  location  of  the  constituents  of  wood  that  are  removed  during 
the  preparation  of  vood  pulp. 


Lignin  is  the  major  component  (28.0  percent)  of  the  part  of  the 
wood  that  is  removed  during  the  manufacture  of  sulphite  pulp.   It  exists  : 
two  forms,—  which  differ  somewhat  in  chemical  composition. 


One  form  is  the  binding  material  (middle-lamella  lignin)  between 
the  wood  fibers;  the  other  is  a  finely  divided  amorphous  material  (cell-wall 
lignin)  in  the  cell  wall.  Whether  the  cell-wall  form  is  chemically  combined 
with  the  cellulose  in  the  wall  or  is  physically  distributed  throughout  is  not 
known  definitely.   If  all  the  constituents  except  lignin  arc  removed  from  a 
transverse  section  of  wood,  the  two  forms  may  be  seen  with  the  aid  of  a  micro- 
scope. One  form,  the  middle  lamella,  is  revealed  as  a  network;  the  other, 
the  cell-wall  form,  appears  as  a  finely  divided  agglomerated  residue  in  the 
space  formerly  occupied  by  the  lignificd  cellulose  in  the  cell  wall.   (Plates 
1  and  2.) 

Further,  if  all  the  constituents  except  lignin  are  removed  from 
transverse  sections,  which  have  been  cut  thicker  than  those  which  were  used 
in  Plates  1  and  2  so  that  the  network  might  remain  intact  during  washing,  the 
two  forms  of  lignin  may  be  separated.  By  impregnating  tho  network  residue 
with  paraffin,  thin  sections  of  the  middle -lame  11a  lignin  may  be  cut  and 
photographed.   (Plates  3  and  4.) 


-Rittcr,  Ind.  Sng.  Chem.  19,  1194  (1925). 
-Published  in  Paper  Trade  Journal,  October  25„  1928. 
R911  -1- 


In  addition,  two  other  groups  of  components,  the  pentosans  not  in 
cellulose  and  the  extractives,  approximately  C  percent  each,  are  a  total  loss 
to  the  manufacturer  of  chemical  pulp  and  paper  and,  still,  further,  the  dif- 
ference between  commercially  "bleached  sulfite  cellulose  and  chlorinated  cellu- 
lose (20  percent  of  the  wood)  is  another  loss  of  lignin-free  material  that  in 
all  probability  is  an  excellent  paper-making  material.   From  work  dene  in  the 
Pulp  and  Paper  Section  of  the  Forest  Products  Laboratory,  it  is  known  that  the 
unbleached  sulfite  pulp  yields  can  be  increased  from  the  usual  40  percent  to 
approximately  50  percent  with  simultaneous  improvement  of  the  quality  of  the 
pulp. 

Lastly,  the  viscose  manufacturer  in  using  the  sulfite  cellulose 
wastes  another  7  percent  of  the  spruce  wood,  leaving  but  34  percent  to  be 
utilized,- 


Constituents  Commonly  Utilized 

Cellulose,  which  is  practically  the  Sole  constituent  of  chemical 
wood  pulp,  consists  of  various  carbohydrates  when  it  is  prepared  from  wood. 
When  isolated  by  chlorination ,  it  constitutes  approximately  60  percent  of  the 
total  wood  forming  the  major  portion  of  the  cell  wall.   In  general,  it  consists 
of  pointed  capsular  fibers.  A  knowledge  of  the  minute  structure  and  the 
properties  of  these  fibers  is  of  importance  to  the  paper  maker,  to  enable  him 
to  best  adapt  his  processes  to  work  in  harmony  with  them  rather  than  against 
them. 


II.  Microstructure  of  the  wood  Fiber 


Separation  of  Layers  in  the  Cell  Wall 

The  cellulosic  material  used  in  examining  the  microstructure  of  the 
wood  fiber  was  prepared  from  thin  longitudinal  wood  shavings.   These  shavings 
were  delignified  by  the  Cross  and  Sevan  method,  were  dehydrated  with  alcohol 
for  several  days,  and  were  kept  in  alcohol  until  required  for  use. 

Examination  of  delignified  fibers  after  they  had  received  alternate 

swelling  and  shrinking  treatments—  with  alkali  and  with  acids  indicated  that 

the  cell  wall  is  in  a  manner  similar  to  that  of  the  cell-wall  layers  of  cot- 

3 
ton-  composed  of  several  layers  packed  together  closely.  These  layers  are  so 

close  together  or  else  are  so  embedded  in  a  cementing  material  of  such  an 

index  of  refraction  that  the  layers  are  invisible  in  the  original  wood.  But, 

by  chemical  and  physical  treatments,  which  the  fibers  receive  through  deligni- 

fication,  swelling,  and  shrinking,  the  binding  material  can  be  removed  or  the 

layers  can  be  pushed  apart,  so  that  the  spacings  become  visible.   The  presence 

of  several  layers  in  the  coll  wall  can  also  be  shown  plainly  by  treating 

delignified  fibers  with  phosphoric  acid, 

—  ■  "' " " 

-On  neutralization  of  the  alkali  with  dilute  acid  the  wood  sections  shrink. 

^Van  Iterson,  Chem.  1/eekblad,  24E,  175  (1927). 
R911  -2- 


If  fibers  that  have  "been  treated  properly  with  alkali  and  acid  are 
examined  with  the  aid  of  a  microscope,  stratifications  that  define  the  various 
layers  can  he  seen  in  the  cell  wall.   (Plate  5.)   Since  the  layers  form 
pointed  capsules  that  are  nested,  complete  layers  can  not  be  separated  by 
sliding  certain  ones  endwise  over  the  others,  even  though  they  may  have  been 
properly  loosened.   If  short  sections  of  such  fibers  are  used,  however,  it  is 
possible  to  remove  the  loosened,  concentric,  tube-like  sections  of  the  layers 
by  sliding  them  endwise.   (Plate  6.) 

At  present,  the  chemical  composition  of  the  binding  material  between 
the  various  layers  in  the  cell  wall  is  unknown.   It  may  be  composed  of  an 
easily  hydrolyzable  portion  of  the  cellulose.  The  quantity  of  this  substance 
may  be  so  minute  that  it  will  be  necessary  to  determine  it  by  difference  in 
the  composition  of  the  residue  before  and  after  its  removal,  rather  than  to 
isolate,  recover,  and  identify  the  binding  material  itself.  This  phase  of 
the  study  will  be  undertaken  later.  The  accomplishment  reported  here  is  the 
actual  separation  of  the  layers  in  the  secondary  thickening  of  the  cell  wall 
in  wood  fibers. 


Orientation  and  Separation  of  the  Fibrils 
in  the  Cell-wall  Layers 

The  swelling  properties  of  bordered  pits  reported  in  Part  III  of 
this  paper,  the  optical  properties  observed  between  ITicol  prisms,  and  the  re- 
sults that  will  be  described  in  Part  II  indicate  that  not  only  do  tiny  fibrils 
form  the  various  layers  of  the  cell  wall  of  wood  fibers,  but  that  these 
fibrils  can  be  separated  by  chemical  means. 

The  fibrils  that  compose  the  outer  layers  are  oriented  at  approxi- 
mately 90°  to  the  long  axis  of  the  wood  fiber.   Immediately  under  the  outside 
layer  of  some  fibers  there  are  occasional  stray  bands  of  fibrils  wound  about 
the  inner  layers  at  approximately  45°  to  the  fiber's  axis.   Such  fibrils  do 
not  form  a  continuous  layer.   In  the  remaining  layers  the  fibrils  are  oriented 
from  0°  to  30°  to  the  axis  of  the  fiber. 

A  study  of  the  orientation  of  the  fibrils  in  the  various  cell-wall 
layers  and  of  the  separation  of  the  fibrils  in  each  layer  was  made  by  two 
methods,  alkali-acid  and  phosphoric  acid  treatments. 

(l)  Alkali-acid  method. — After  alternate  alkali  and  acid  treatment 
of  fibers,  faint  stratifications  became  prominent  and  more  and  more  striations 
became  visible.   In  places  at  which  the  outer  layer  had  been  dissolved,  there 
was  extreme  outward  swelling  of  the  inner  layers.   Such  swelling  made  apparent 
the  pronounced  constrictions  at  the  places  where  the  outer  layer  was  still 
intact,  and  also  the  less  prominent  constrictions  on  the  fibers  that  had  stray 
fibril  bands  wound  about  their  inner  layers  at  an  angle  of  45°.   (Plate  7.) 
Continued  treatment,  v/ith  the  aid  of  slight  pressure  on  the  cover  glass, 
broke  the  constricting  bands  and  the  inner  layers  separated  into  fibril  bun- 
dles.  (Plate  8.)   It  was  possible  to  separate  these  bundles  into  individual 
fibrils,  which  naturally  have  a  diameter  smaller  than  that  of  the  bundles. 
Unfortunately,  no  satisfactory  photomicrographs  were  obtained. 

R911  -3- 


(2)  Phosphoric  acid  method. — It  is  possible  to  control  the  reaction 
of  the  phosphoric  acid  method  upon  cellulose  better  than  that  of  the  alkali- 
acid  method.   Further,  it  is  possible  to  reveal  the  minute  structural  arrange- 
ment of  the  layers  in  a  manner  that  can  not  be  accomplished  oy   the  alkali- 
acid  method.  Phosphoric  acid  seems  to  have  a  specific  property  for  develop- 
ing striations  in  wood  fibers  by  loosening  the  layers  and  the  fibrils  before 
the  skeleton  structure  dissolves. 

Fibers  treated  by  the  phosphoric  acid  method  shov;  that  solution  of 
the  outer  layer  at  intervals  is  accompanied  by  extreme  outward  swelling  of 
the  inner  layers;  such  fibers  are  constricted  at  the  places  where  the  outer 
layer  is  still  intact.   (Plate  9.)  Extremely  high  magnification  shows  that 
the  cell-wall  layers  separate  in  the  transverse  direction.   (Plate  10.)   Such 
photographs  suggest  that  the  orientation  of  the  fibrils  in  the  outer  and  in 
the  inner  layers  is  radically  different. 

Through  slowly  dissolving  the  outer  layer,  it  became  apparent  that 
striation  and  separation  of  the  fibrils  precede  the  ultimate  solution  of  the 
layer  as  an  entirety.   Since  the  fibrils  in  the  outer  layer  are  oriented  at 
approximately  90°  to  the  axis  of  the  wood  fiber  (pi.  11) ,  it  is  obvious  that 
the  fibers  can  not  swell  outwardly  beyond  the  maximum  limits  that  they  assume 
in  a  water  medium,  unless  the  fibrils  in  the  outer  layer  expand  lengthwise  or 
break.   Such  a  structure  also  accounts  for  delignified  fibers  swelling  inwardly 
when  the  outer  layer  is  still  intact.   (Plate  18.) 

On  account  of  the  convexity  of  the  surface  of  the  bead-like  swell- 
ings in  Plate  10,  the  minute  structure  of  the  inner  layers  is  not  visible.  A 
swollen  surface  both  flatter  and  longer  must  be  examined  if  the  tiny  fibrils 
are  to  be  seen  in  their  proper  orientation.  3y  proper  focusing  of  the  micro- 
scope the  orientation  of  the  fibrils  in  the  various  layers  can  be  studied. 
(Plate  12.)   If  the  acid  treatment  is  continued,  the  fibrils  are   loosened  to 
a  greater  extent.   (Plate  13.)   3y  allowing  the  reaction  to  proceed  still 
further,  the  individual  fibrils  are  isolated.   (Plate  14.) 

If  the  minute  structural  arrangement  of  the  outer  layer  is  contrasted 
with  that  of  the  inner  layers,  it  will  be  seen  that  wood  fibers  are  designed 
to  withstand  both  transverse  and  longitudinal  stresses. 

This  separation  of  the  cell  wall  of  the  wood  fiber  into  fibrils 
confirms  some  findings  of  Waentig.— 

Separation  of  the  "Fusiform  Bodies"  in  the  Pibriis 

A  careful  examination  of  the  isolated  fibrils  from  the  inner  layers 
of  the  cell  wall  under  the  high  power  of  the  microscope  revealed  that  they 
were  made  up  of  units,  the  ends  of  which  taper  to  sharp  points;  the  units  are 
held  together  by  a  slight  overlapping  of  the  pointed  ehds5  so  as  to  form  the 

T  ~ ' 

-Waentig.  Papierfabrikant  25,  115  (1927), 

— These  units  differ  from  the  dermatosomes  described  by  Wiesner  in  his 
Elcmentar  Structur,  Alfred  Holder  *  Wien,  1892,  p.  162. 

R911  _4_ 


Lc> 


tiny,  slender  fibrils  of  a  diameter  practically  uniform  throughout  their  entire 
length.  The  long  axis  of  each  unit  is  parallel  to  the  long  axis  of  the  fibril. 
The  reaction  of  the  phosphoric  acid  under  proper  conditions  slowly  opens  up 
the  natural  planes  of  cleavage  "between  the  tiny  units,  which  are  of  fusiform 
shape.   (Plate  15.)   Since,  as  far  as  the  senior  author  knows,  these  are  newlj 
discovered  units,  which  have  "been  separated  and  photographed  for  the  first 
time  in  the  investigation  now  reported,  they  have  "been  given  the  descriptive 
name  "fusiform  bodies. " 


III.   Properties  of  Wood 


Physical  Pro-perties 

Some  of  the  swelling  properties  of  wood  have  "been  known  for  a  long 
time  through  everyday  experiences  with  the  increased  external  dimensions  that 
are  produced  in  wood  products  when  they  are  changed  from  dry  and  wet  condi- 
tions "by  soaking  in  liquids,  such  as  water,  and  solutions  of  acids  and  alka- 
lis. Water  swells  dry  wood  approximately  to  its  green  volume.   Strong  acids 
and  alkalis  swell  dry  wood  beyond  its  green  volume. 

With  the  aid  of  a  microscope,  it  may  be  seen  that  the  cell  walls  of 
wood  also  swell  internally,  and  that  the  internal  swelling,  when  an  alkali  or 
a  strong  acid  is  used  as  the  reagent,  may  be  sufficiently  severe  to  fill  the 
cell  cavities.   If  a  section  of  wood  in  its  original  state  is  swollen,  the 
fibers  retain  their  external  shape,  which  in  cross  section  shows  definite 
angles.   (Plates  16  and  17.) 

Prom  Plate  17  it  is  evident  that  a  swollen  condition  retards  the 
movement  of  impregnating  liquors,  prior  to  cooking,  through  the  pits  and  the 
cell  cavities.  On  the  other  hand,  a  condition  such  as  that  shown  in  this 
plate  indicates  that  the  fibers  have  been  impregnated  by  diffusion  of  the 
alkali  through  the  actual  cell  walls.  The  appearance  of  the  section  shown 
suggests  that  impregnation  of  wood  chips  with  alkali  liquors  takes  place 
principally  by  diffusion  through  the  cell  wall  itself  rather  than  through  the 
various  natural  openings  in  the  wall. 

Acid  solutions  of  ordinary  concentration  produce  very  little  swell- 
ing of  the  cell  wall  b: - ond  its  green  volume.  The  original  sizes  of  the  open- 
ings, therefore,  remain  practically  unaltered.   Such  a  condition  suggests 
that  impregnation  of  wo  C  chips  with  an  acid  cooking  liquor  take3  place  prin- 
cipally through  the  pi  b«3  and  cell  cavities. 

If  delighifisd  fibers  are  treated  with  alkali  or  with  strong  acids, 
they,  too,  swsl]  by  crowding  the  cell  wall  into  the  lumen,  but  their  cross- 
sed     area  is  changed  from  a  polygonal  to  a  circular  shape.   (Plate  18.) 
On  reacting  with  the  drastic  reagents,  the  fibers  become  slightly  plastic  and 
they  tend  to  assume  shapes  that  have  the  minimum  external  surface  in  both  the 
transverse  and  the  longitudinal  dimensions. 


R911  -5- 


By  alternately  swelling  wood  fibers  beyond  their  green  volume  and 
then  shrinking  then  quickly,  markings  are  developed  that  suggest  the  minute 
microstructure  described  in  Part  II  of  this  paper. 

Sodium  hydroxide  solution  (15.6  percent  concentration)  was  used  for 
swelling  the  fibers  shown  in  Plate  18.  The  appearance  of  those  fibers  gives 
an  idea  of  the  appearance  of  wood  fibers  in  cross  section  after  the  alkaline 
treatment  in  the  viscose  process,  and  also  in  the  alpha-cellulose  determina- 
tion. The  numerous  pit  openings  of  the  swollen  cell  walls,  which  do  not  ap- 
pear in  the  photomicrograph,  are  changed  to  oblong  slits  that  are  practically 
closed. 

Optical  properties  that  suggest  the  structure  of  the  cell  wall 
about  the  bordered  pits  are  manifested  when  the  pits  are  examined  in  polarized 
light.   It  lias  been  knownS.  for  a  long  time  that  the  secondary  layer  of  the 
cell  wall  rotates  the  plane  of  polarized  light  and  that  the  face  of  a  bordered 
pit  shows  the  commonly  observed  dark  cross  when  it  is  placed  betv/een  Mcol 
prisms  that  are  crossed  at  90°.   The  optical  properties  of  the  secondary  layer 
are  commonly  considered  to  be  due  to  an  orderly  arrangement  of  cellulose  mole- 
cules in  chains.!  (Fageli '  s  hypothesis);  X-ray  diagrams  of  Sponsler  and  Doreii 
suggest  that  these  chains  are,  in  general,  parallel  to  the  longitudinal  axis 
of  the  fiber.  The  fibrils  in  the  secondary  layer  about  the  pit  are  bent 
around  the  opening,  making  their  arrangement  somewhat  circular.  The  fibrils 
in  the  outside  layer  are  present  and  arc  oriented  at  90°  to  the  fiber's  axis. 
Bending  around  the  opening  they  superpose  a  layer  of  concentric  rings  over 
the  slight  distortions  in  the  circular  structure  of  the  inner  layers.   It  is 
because  of  this  involved  total  structure  that  the  bordered  pits  exhibit  a 
symmetrical  dark  cross  through  a  complete  rotation  of  the  microscope  stage. 
(Plates  21  and  22.) 


IV.   Significance  of  Fiber  Microstructure 
to  Chemical  Pulping 

Treatments  that  tend  to  separate  wood  fibers  into  fibrils  and,  in 
turn,  tend  to  separate  the  fibrils  into  the  fusiform  bodies,  are  of  interest 
to  the  paper  maker.   If  a  definite  percentage  of  the  fibers  in  a  pulp  are  in 
a  physical  condition  similar  to  the  conditions  in  Plates  5,  6,  7,  11,  and  12, 
it  may  aid  immensely  in  the  felting  qualities  of  the  pulp.  On  the  other  hand, 
if  the  reaction  should  be  carried  on  sufficiently  to  put  a  large  percentage 
of  the  fibers  in  the  condition  shown  in  Plate  13,  the  pulp  might  be  useless 
for  making  paper. 


c 

-Dippcl,  "Das  Mikroskop,"  2,  p.  264. 
7 

-Sachs,  "History  of  Botany,"  p.  350. 
8 
"Fourth  Colloid  Symposium,"  Monograph,  p.  174  (1926) 


R911  -6- 


Further,  from  the  results  already  presented  in  this  paper,  it 
appears  that  pulps  of  different  qualities  and  varying  yields,  produced  "by 
different  cooking  conditions,  should  show  some  difference  in  the  microstruc- 
ture  of  the  fibers.  Also,  pulps  cooked  under  the  more  drastic  of  the  usual 
commercial  conditions  might  respond  more  readily  to  the  treatments  previously 
described  than  pulps  cooked  under  milder  conditions.   In  addition,  pulps 
beaten  for  varying  periods  might  show  a  tendency  to  respond  to  the  acid  treat- 
ments more  readily  as  the  beating  time  increased. 

^The  physical  properties  of  the  wood  fibers  determine  to  a  large 
degree  the  ease  with  which  phosphoric  acid  reacts  with  the  cell  wall.  A 
slight  rupture  of  the  woody  tissues,  such  as  frayed  ends,  aids  in  starting 
the  reaction.  This  fact  may  be  demonstrated  by  treating  short  sections  of 
fibers  with  the  acid.  With  such  a  section,  the  solution  of  the  outer  layer 
begins  at  the  frayed  ends,  and  progresses  toward  the  middle.  By  arresting 
the  reaction  before  all  the  outer  layer  is  removed,  it  is  possible  to  obtain 
a  residue  that  consists  of  loosened  bundles  tied  with  the  spiral  bands  that 
form  the  remainder  of  the  outer  layer.   (Plate  19.) 


Optical  Properties 

Isolated  "fusiform  bodies,"  fibrils,  and  fibril  bundles  between 
Nicol  prisms  exhibit  the  same  property  as  wood  fibers,  in  that  they  transmit 
polarised  light  when  they  are  oriented  at  an  angle  to  the  axes  of  the  crossed 
prisms,  but  do  not  do  so  when  oriented  parallel  to  either  of  the  axes. 
(Plate  20.) 

The  bead-like  swelling  shown  in  Plate  10,  if  placed  between  ITicol 
prisms,  exhibit  a  "dark  cross"  when  the  axis  of  the  fiber  is  parallel  to  the 
axis  of  one  of  the  prisms.  When  the  microscope  stage  is  rotated  45°,  the  dark 
cross  becomes  slightly  distorted.  The  bead-like  swellings  disappear,  in  gen- 
eral, as  spherical  bodies  with  a  slight  distortion  in  the  direction  of  the 
fiber's  axis.  A  cross  section  of  such  a  body  is  composed  of  an  approximate 
circle  made  up  of  concentric  rings  of  visible  fibrils,  which  are  distorted  in 
a  manner  similar  to  that  of  the  swellings.  With  such  a  structure,  the  opti- 
cal phenomenon  of  the  swellings  can  be  explained. 

Some  tests  were  made  to  determine  whether  such  relationships  could 
be  shown.  Preliminary  experimental  work  was  done  on  two  series  of  sulfite 
cooks  of  white  spruce  and  Eastern  hemlock,  respectively.  Each  series  consisted 
of  a  pulp  showing  high  strength  and  high  yield  in  contast  to  one  showing  low 
strength  and  low  yield.  These  pulps  were  subjected  to  the  Laboratory  standard 
strength-development  procedure  by  use  of  the  ball  mill. 

The  bleachabilities  of  the  pulps  were  also  determined.   The  essential 
data  are  recorded  in  Table  1. 

The  differences  in  the  two  spruce  pulps  are  greater  than  those  in 
the  hemlock  pulps.   In  maximum  bursting  strength,  the  second  spruce  pulp  is 
0.55  point  higher  than  the  first;  the  yield  is  8.6  percent  higher.  The  maxi- 
mum bursting  strength  of  the  Becond  hemlock  pulp  is  0.21  point  higher,  while 
the  yield  is  2.9  percent  higher  than  the  first , 

R911  <-7~ 


The  spruce  pulns  were  stained  with  Bismarck  brown,  air  dried,  treated 
with  a  solution  of  phosphoric  acid,  heated  for  4  minutes  at  60°  C. ,  and  cooled. 
Slides  were  then  made  and  photomicrographs  taken.  The  hemlock  pul'os  were 
treated  in  the  same  way  except  that  they  were  heated  for  3  minutes  instead  of 
4.   In  addition,  4  of  the  initial  samples  were  treated  with  a  slightly  stronger 
acid,  at  room  temperature,  to  show  more  clearly  the  differences  in  the  fibers 
"before  milling.  The  photomicrographs  of  these  fibers  appear  in  Plates  26,  27, 
28,  and  29. 

Untreated  fibers,  "both  milled  and  not  milled,  of  Pulp  3236-1  are 
shown  in  Plates  23,  24,  and  25.   Piters  of  the  4  pulps,  milled  and  not  milled 
and  treated  with  phosphoric  acid,  are  shown  in  Plates  3C  to  49,  inclusive. 

The  results  show  that  pulps  prepared  under  mild  cooking  conditions 
are  less  susceptible  to  the  attack  of  phosphoric  acid  than  those  prepared  by 
drastic  cooking  conditions.   Differences  in  the  susceptibility  to  the  attack 
appear  when  plates  prepared  from  the  two  unmilled  spruce  pulps  and  the  two 
unmilled  hemlock  pulps  are  compared.   Por  example,  contrast  Plates  26  and  27; 
28  and  29;  30  and  31;  and  40  and  41. 

The  results  further  show  that  the  binding  material  and  the  helical 
winding  of  fibrils  forming  the  outer  layer  of  the  fiber  have  been  partially 
or  wholly  dissolved,  allowing  the  inner  portion  of  the  fiber  to  expand.   In 
some  cases,  the  inner  fibrils  may  be  seen  slightly  separated,  forming  an  ex- 
tended helix. 

The  effect  of  the  acid  is  also  noticeable  as  the  milling  progresses. 
When  the  outer  portion  of  the  fiber  has  been  ruptured  mechanically,  the  inner 
part  is  attacked  by  the  acid  at  the  rupture  and  swelling  takes  place.   In  the 
refined  pulps,  also,  the  stronger  pulp  shows,  in  general,  less  effect  of  the 
acid. 

By  treating  fibers  from  various  sulfite  pulps  with  phosphoric  acid 
and  examining  them  under  the  microscope,  it  is  possible  to  observe  differences 
in  the  quality  of  the  pulp.  Just  how  fine  a  distinction  can  be  made  remains 
to  be  worked  Otvb.   It  may  be  possible  to  evaluate  the  pulp  numerically  by  using 
phosphoric  acid  solutions  of  different  concentrations,  noting  the  strength  at 
which  the  pulp  is  attacked. 

Although  a  rapid  qualitative  test  may  be  developed  from  the  method, 
more  important  is  the  information  it  gives  on  the  fundamo;  tal  relation  of  the 
microstructurc  of  the  fiber  to  different  cooking  condit ions,  yields,  and 
strength  properties. 

Summary 

The  locatiqn  in  the  wood  of  the  two  forms  of  lignin  is  described. 
The  two  forms  are  shown  in  photomicrographs. 


R911 


The  possibility  of  obtaining  a  yield  of  60  percent  of  lignin-free 
fibers  for  paper  material  is  suggested. 

The  cell  wall  of  wood  fibers  is  composed  of  several  layers,  which 
can  be  separated  by  chemical  means. 

The  layers  in  the  cell  vail  of  a  wood  fiber  can  be  separated  into 
fibrils  by  chemical  means.  The  fibrils  in  the  outer  layer  are  oriented  at 
approximately  right  angles  to  the  fiber's  axis,  while  those  in  the  remaining 
layers  are  from  0°  to  30°  thereto. 

The  fibrils  can  be  separated  into  regularly  shaped  "fusiform  bodies" 
with  optical  properties  similar  to  those  of  the  fibrils. 

When  either  lignified  or  delignified  wood  fibers  are  treated  with 
swelling  reagents,  the  fiber  walls  thicken  outwardly  and  also  inwardly.  The 
polygonal  shape  of  the  cross  section  of  delignified  fibers  is  unaltered,  but 
the  cross  section  of  delignified  fibers  is  limited  by  the  outer  layer  of 
fibrils,  which  are  oriented  at  90°  to  the  fiber's  axis. 

The  optical  phenomenon,  when  bordered  pits  are  observed  between 
Mcol  prisms,  is  explained  on  the  basis  of  the  ring-like  structural  arrange- 
ment of  the  cellulosic  material  of  the  cell  wall. 

The  effect  of  phosphoric  acid  on  pulps  obtained  from  two  series  of 
cooks  of  spruce  and  hemlock  is  described.   Its  effect  is  more  severe  on  the 
pulps  from  the  more  drastic  cooks,  both  in  the  raw  and  refined  condition. 
The  effect  increases  as  the  period  of  milling  increases. 

The  swelling  and  dissecting  action  of  the  phosphoric  acid  on  the 
fibers  is  explained  on  the  assumption  that  part  of  the  outer  layer  and  more 
of  the  building  material  between  the  fibrils  in  the  various  layers  of  the  cell 
wall  are  removed  by  the  more  drastic  cooking  conditions.  Milling  has  the 
mechanical  effect  of  progressively  rupturing  the  outer  layer  of  fibrils  and 
of  loo-Faring  the  inner  fibrils.   Such  an  effect  permits  a  more  rapid  attack 
by  the  phosphoric  acid. 

Is  suggested  that  the  phosphoric  acid  treatment  developed  in  the 
study  cLiseuascvd  in  this  paper  may  be  further  standardized  to  provide  a  new 
method  for  Vao   evaluation  of  pulp  quality. 


R911  -9- 


Legends  for  Plates   on  Following  Pages 

Plate     1* — The  middle -lame 11a  lignin  and  the  cell-wall   lignin  of  red  alder. 

Plate     2. — Another  transverse  section  of  red  alder  which  also   shows  the  two 

forms  of  lignin.  Some  of  the  middle-lamella  lignin  is  slightly 
out  of  focus  "because  of  making  visible  larger  quantities  of  the 
cell-wall   lignin. 

Plate     3, — A  cross   section  cut  from  a  block  of  yellow  pine  middle-lamella 
lignin  which  was  first   impregnated  with  paraffin.      The  rough 
appearance   of  the  paraffin  is  due   to  a  slight   melting  and  re- 
solidifying of  the  paraffin  on  the   surface. 

Plate     4. — A  cross   section  of  yellow  pine   middle-lamella  lignin  similar   to 

that  of  Plate  3.  The  section  shows  that  the  cellulose  and  the 
cell-wall  lignin  can  he  removed  with  very  little  injury  to  the 
middle   lamella. 

Plate     5. — Shows   a  separation  of  the   delignified  cell  wall  of  elm   into  four 

distinct  layers  "by  means  of  a  68  percent  solution  of  phosphoric 
acid. 

Plate     6. — Short   sections  of  delignified  elm  fibers    in  which  the   cell  wall 
layers  have  been  separated  and  slipped  endwise. 

Plate     7. — Delignified  elm  fiber  which  has  received  alternated  treatments 

with  alkali  and  acid.     The  outer  layer  has  been  removed  from  a 
large  portion  of   the  fiber.     A  helical  band  at   approximately 
45°  keeps  the   cell  wall  from  rupturing. 

Plate     8. — Delignified  elm  fiber   treated  alternately  with  alkali   and  acid. 
Shows  a  separation  of   the  cell  wall  into  fibril  bundles. 

Plate     9. — Shows   the  transverse  swelling  of   the    inner  layers   of  elm  in  places 
at  which  the   outer  layer  has  been  dissolved. 

Plate   10. — Shows  three   separate  layers  of  a  delignified  elm  fiber   at   the   con- 
stricted places  and  the   transverse   swelling  of  two   inner   layers. 

Plate   11.— Section  of  the   outside   layer,    showing  helical   striations  extend- 
ing around  the  fiber  at   right   angles   to   the    fiber's   axis. 

Plate    12. — Shows  minute   fibrils   of  the   inner  cell  wall   layers.     The  fibrils 
have  been  loosened  by  phosphoric   acid  treatment. 

Plate   13. — Shovfs  appearance   of  a  .fiber   after  the  fibrils   have  been  well 
loosened. 

Plate   14. — Shows   a  more   nearly  complete   separation  of  the   cell  wall  layers 
into  fibrils. 


R911  -10- 


Plate  15. — Shows  how  the  "fusiform"  bodies  in  the  fibrils  can  be  separated. 

Plate  16. — Cross  section  of  Western  yellow  pine  soaked  in  water.   Note  the 
general  rectangular  shape  of  the  cells. 

Plate  17. — Cross  section  of  Western  yellow  pine  which  has  been  swollen  with  15 
percent  alkali.  Note  the  puffy  appearance  of  the  surface,  the 
thickening  of  the  cell  wall,  and  the  general  rectangular  shape 
of  the  cells, 

Plate  18. — Cross  section  of  Western  yellow  pine  which  has  been  delignified  so 

as  to  obtain  isolated  cells.   On  treatment  with  15  percent  alkali 
the  isolated  cells  assume  a  cylindrical  shape  with  the  lumen 
closed, 

Plate  19. — Short  sections  of  delignified  elm  fibers,  showing  the  bundle-like 
residue  obtained  when  the  dissolving  action  of  phosphoric  acid 
is  arrested  before  the  outer  layer  is  removed  completely. 

Plate  20. — Sho\7S  that  the  fibrils  between  ITicol  prisms  are  luminous  when 
oriented  at  an  angle  to  the  axis  of  either  ITicol  prism,  but 
dark  when  parallel  thereto. 

Plate  21. — Radial  face  of  Western  yellow  pine.   The  fibers  are  oriented  par- 
allel to  the  axis  of  one  Nicol  prism.  The  fibers  are  dark; 
lines  of  the  "dark  cross"  are  parallel  to  the  corresponding  axes 
of  the  ITicol  prisms. 

Plate  22. — Radial  face  of  Western  yellow  pine  between  ITicol  prisms.  The 
fibers  are  oriented  at  approximately  45°  to  the  axes  of  the 
Ficol  prisms.  The  fibers  arc  luminous;  lines  of  the  "dark  cross" 
are  parallel  to  the  corresponding  axes  of  the  ITicol  prism. 

Plate  23. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  not  milled. 

Plate  24. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  milled  40  minutes. 

Plate  25. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  milled  80  minutes. 

Plate  26. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  not  milled;  treated 
with  phosphoric  acid. 

Plate  27. — Spruce  sulphite  pulp.  Pulp  3226;  low  yield;  not  milled;  treated 
with  phosphoric  acid. 

Plate  28. — Hemlock  sulphite  pulp.  Pulp  3317;  high  yield;  not  milled;  treated 
with  phosphoric  acid. 

Plate  29. — Hemlock  sulphite  pulp.  Pulp  3314;  low  yield;  not  milled,  treated 
with  phosphoric  acid. 

Plate  30. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield,  not  milled;  treated 
with  phosphoric  acid. 

R911  -11- 


Plate  31. — Spruce  sulphite  pulp.   Pulp  3226;  lev;  yield;  not  milled;  treated 
with  phosphoric  acid. 

Plate  32. — Spruce  sulphite  pulp.   Pulp  5236;  high  yield;  milled  20  minutes; 
treated  with  phosphoric  acid. 

Plate  33. — Spruce  sulphite  pulp.  Pulp  3226;  low  yield;  milled  20  minutes; 
treated  with  phosphoric  acid. 

Plate  34. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  milled  40  minutes; 
treated  with  phosphoric  acid. 

Plate  35. — Spruce  sulphite  pulp.   Pulp  3225;  low  yield;  milled  40  minutes; 
treated  with  phosphoric  acid. 

Plate  32. — Spruce  sulphite  pulp.   Pulp  3236;  high  yield;  milled  60  minutes; 
traated  uith  phosphoric  acid. 

Plate  57. — Spruce  sulphite  pulp,   Pult  3226;  low  yield;  milled  60  minutes; 
treated  uith  phosphoric  acid. 

Plate  35. — Spruce  sulphite  pulp.  Pulp  3236;  high  yield;  milled  80  minutes; 
treated  with  phosphoric  acid. 

Plate  59. — Spruce  sulphite  pulp.   Pulp  5225;  low  yield;  milled  80  minutes; 
treated  with  phosphoric  acid. 

Plate  40. — Hemlock  sulphite  pulp.  Pulp  5517;  high  yield;  not  milled;  treated 
with  phosphoric  acid. 

Plate  41. — Hemlock  sulphite  pulp;   Pulp  5314;  low  yield;  not  milled;  treated 
uith  phosphoric  acid. 

Plate  42. — Hemlock  sulphite  pulp.  Pulp  3317;  high  yield;  milled  20  minutes; 
treated  with  phosphoric  acid. 

Plate  43. — hemlock  sulphite  pulp.   Pulp  5514;  low  yield;  milled  20  minutes; 
treated  with  phosphoric  acid. 

Piste  44. — Hemlock  sulphite  pulp.   Pulp  5317;  high  yield;  milled  40  minutes; 
treated  with  phosphoric  acid. 

Plate  45. — Hemlock  sulphite  pulp.  Pulp  5514;  low  yield;  milled  40  minutes; 
treated  with  phosphoric  acid. 

Plate  45. — Hemlock  sulphite  pulp.  Pulp  5317;  high  yield;  milled  60  minutes; 
treated  with  phosphoric  acid. 

Plate  47. — Hemlock  sulphite  pulp.  Pulp  3314;  low  yield,  milled  50  minutes; 
treated  with  phosphoric  acid. 

Plate  48. — Hemlock  sulphite  pulp.   Pulp  3317;  high  yield;  milled  80  minutes; 
created  with  phosphoric  acid. 

Plate  49. — Hemlock  sulphite  pulp.   Pulp  3514;  lev/  yield;  milled  80  minutes; 
treated  with  phosphoric  acid. 

R911  -12- 


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R911 


UNIVERSITY  OF  FLORIDA 


3  1262  08926  9905 


